To increase the useful life of a rotary cone drill bit, engineers have developed support arm-cutter assemblies that reduce or eliminate the amount of borehole debris that contacts the inner seal and clean the area outside the seal gland. The inner seal is typically formed from an elastomeric material and is disposed in an inner seal gland to form a fluid barrier between the borehole and the bearing-surface regions within the cone cutter cavity. Debris, such as fine cuttings generated while drilling, within the inner seal gland, often wears against the elastomeric seal and its mating surface as the cutter rotates about the support-arm spindle. Over a period of use, the contacting debris wears the seal and the mating surface sufficiently to gain entrance into the bearing-surface regions. The debris then wears against the bearing surfaces, decreasing the lifetime of the drill bit. Therefore, reducing the amount of borehole debris that enters the inner seal gland often increases the useful life of the rotary cone drill bit.
Conventional air-environment arm-cutter assemblies, such as those used for the formation of blast holes, direct a gas, such as air, into the arm-cutter gap between the backface of the cutter and the last machined surface of the support arm to reduce the amount of debris that contacts the inner seal. The gas is directed to flow out of the gap and into the borehole to reduce the amount of debris entering the inner seal gland.
U.S. Pat. No. 4,183,417, issued to Levefelt and entitled Roller Bit Seal Excluded From Cuttings By Air Discharge, discloses a rotary roller bit having an arm-cutter assembly that discharges air into the arm-cutter gap through an annular air chamber. The air flows from the gap and into the borehole to reduce the amount of debris that contacts a seal ring. However, the last machined surface, the backface, or both may wear during operation of the rotary bit. This wearing may cause the gap to widen, increasing the cross-sectional area through which the air flows and reducing the blowing force of the air flow. Such a force reduction often decreases the effectiveness of the air flow in reducing the amount of debris that enters the gap and contacts the seal ring.
U.K. Patent No. 2,019,921, issued to Schumacher and entitled Drill Bit Air Clearing System, discloses an earth boring drill having an arm-cutter assembly that discharges air directly into a lone seal gland such that the air flows through the gap and into the borehole. However, the air flow undergoes an abrupt 90 degree shift in direction because the discharge passage is substantially perpendicular to the gap. This shift causes turbulence that may reduce the effectiveness of the air flow in reducing the amount of debris entering the seal gland. Moreover, the passage discharges the air toward the gap side of the seal, such that debris entering the seal gland may be forced against the seal. As discussed above, this debris may wear the seal, enter the bearing-surface regions, and reduce the useful life of the drill bit.
A variation of the arm-cutter assembly of U.K. Patent No. 2,019,921 has a discharge passage that directs air perpendicularly into the gap instead of the seal gland. An annular, metal seal forms a debris barrier between the gap and the seal gland. As discussed above, air flow turbulence caused by the 90 degree direction shift may reduce the effectiveness of the air flow in reducing the amount of debris that enters the seal gland through the gap and may force debris against the metal seal. This debris may wear or otherwise circumvent the metal seal, enter the seal gland, wear the inner seal, and enter the bearing-surface regions.
U.S. Pat. No. 4,981,182, issued to Dysart and entitled Sealed Rotary Blast Hole Drill Bit Utilizing Air Pressure For Seal Protection, discloses an inner seal in an inner seal gland, an outer seal that divides an outer seal gland into two regions, and a porous gas restrictor between the outer seal gland and the gap.